S.M. Niebur1, W.R. Binns1, E.R. Christian2, A.C. Cummings3, J.S. George3, 
P.L. Hink1, J. Klarmann1, R.A. Leske3, M. Lijowski1, R.A. Mewaldt3, E.C. Stone3, 
T.T. von Rosenvinge3, M.E. Wiedenbeck4, N.E. Yanasak4

1 Washington University, St. Louis, Missouri, USA
2 Goddard Space Flight Center, Greenbelt, Maryland, USA
3 California Institute of Technology, Pasadena, California, USA
4 Jet Propulsion Laboratory, Pasadena, California, USA

We report the first observations of galactic cosmic rays that can be used to directly study energy-dependent electron capture of secondary isotopes. Secondary nuclei that decay only by electron capture can be used to study the energies at which galactic cosmic rays propagate through the interstellar medium. Electron-capture decay occurs predominantly at low energies where there is a significant probability of electron attachment. Consequently, propagation models which incorporate energy-dependent electron attachment cross sections predict that the abundances of these decay nuclei will be depleted and the corresponding abundances of their daughter nuclei will be elevated at low energies. Evidence of energy-dependent electron-capture decay over and above that expected for a standard leaky-box propagation (excluding reacceleration) could provide evidence of reacceleration of galactic cosmic rays. The 51Cr --> 51V and 49V --> 49Ti decays are particularly suitable for this study, since the amount of electron-capture decay expected for 51Cr and 49V varies significantly over the interstellar energy interval measured by the Cosmic Ray Isotope Spectrometer (CRIS) on ACE. These measurements and additional modeling calculations will be used in the future to study the possible effects of reacceleration on cosmic rays.